1. Field of the Invention
The present invention relates to thin-film superconductive devices and more particularly to thin-film superconductive devices having superconductive films of varying volume per unit length.
2. Description of the Prior Art
Considerable research has been devoted to studying the properties of thin-film superconductors. In particular attention is directed to U.S. Pat. No. 3,691,381 to Kleppner, issued Sept. 12, 1972. This patent discloses a low energy superconducting particle counter including a superconductive thin-film in thermal contact with a sapphire substrate. A bias current is passed through the film to maintain the film at an operating point just below its critical current. Incident particles of sufficient energy induce regeneration causing the entire film, or at least a portion of the film, to go normal resulting in a IR drop across the film which produces a detectable output pulse.
This reference describes a first system in which an entire film is driven normal by impinging particles, at which time the film can be returned to its superconducting state only by briefly interrupting the bias current. The reference also describes in much less detail a system which does not require an entire film to be driven to its normal state, and which operates with only a portion of the film being driven normal by impinging particles. The latter technique involves the use of a persistent current in a superconducting loop. The magnetic energy stored inductively in the loop may be monitored, and dissipation of the magnetic energy would indicate that the loop has been driven out of the superconducting state by the impingement of a particle.
The device disclosed in the Kleppner patent, although useful in many applications, is sensitive to the energy of impinging particles and other factors and thus is not suitable for digital applications. Furthermore, the device disclosed in the Kleppner reference lacks parameters which can be adjusted to modify or regulate its performance for various different types of applications. In addition, although the Kleppner apparatus is described as a "low energy" particle counter, the apparatus is in reality incapable of detecting particles having energies below the Mev range. Thus it is not capable of detecting particles of extremely low energy.
A superconducting thin-film detector is also described in a paper by Crittenden and Spiel published in the Journal of Applied Physics, Vol. 42, No. 8, pp. 3182-3188 (July 1971). This device is quite similar to that disclosed in the Kleppner patent and it does not include any features which would eliminate the above-mentioned deficiencies in the Kleppner apparatus. In particular, both the Crittenden and Spiel article and the Kleppner patent appear to assume the desirability of providing a thin-film superconductor of essentially constant width along its entire length. The present invention, as will be explained subsequently, does not rely upon this assumption and in fact utilizes a thin-film superconductor of varying volume per unit length in which the volume variations are controlled to provide greater particle detecting sensitivity and controlled digital output signals.